Process for producing a synthesis gas

ABSTRACT

Process for manufacturing a hydrogen-containing synthesis gas from a natural gas feedstock, comprising the conversion of said natural gas into a raw product gas and purification of said product gas, the process having a heat input provided by combustion of a fuel; said process comprises a step of conversion of a carbonaceous feedstock, and at least a portion of said fuel is a gaseous fuel obtained by said step of conversion of said carbonaceous feedstock.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/524,845, filed May 5, 2017, which is a national phase ofPCT/EP2015/072992, filed Oct. 6, 2015, and claims priority to EP14192002.5, filed Nov. 6, 2014, and EP15162906.0, filed Apr. 9, 2015,the entire contents of all of which are hereby incorporated byreference.

FIELD OF APPLICATION

The invention concerns a process for producing a hydrogen-containingsynthesis gas from a natural gas feedstock, a related plant and a methodof revamping of related plants. The invention relates in particular toproduction of ammonia synthesis gas comprising hydrogen and nitrogen inabout 3:1 molar ratio.

PRIOR ART

The production of synthesis gas from a hydrocarbon feedstock usuallyinvolves a combined reforming process in which a primary reformer is fedwith desulphurized hydrocarbons and steam and a secondary reformerreceives the partially reformed gas from the primary reformer and a flowof a suitable oxidant, for example air or oxygen.

The reformed gas exiting the secondary reformer is then typicallytreated in a series of down-stream equipment items to obtain a synthesisgas with a composition suitable for a specific use.

For example, the synthesis of ammonia (NH₃) requires a synthesis gascomprising hydrogen (H₂) and nitrogen (N₂) in a suitable molar ratio ofabout 3:1, the so called make-up gas. The term ammonia syngas iscommonly used with reference to a synthesis gas with the abovecomposition.

The ammonia syngas is generally produced in a front-end section and theconversion of said synthesis gas into ammonia is carried out in asynthesis loop.

From an efficiency standpoint, the ideally suited hydrocarbon feedstockfor the manufacture of ammonia syngas is natural gas, because it ischaracterized by one of the highest contents of hydrogen among allfossil fuels.

An example of process for the production of ammonia syngas starting fromnatural gas is disclosed in EP 2 065 337.

The production of ammonia syngas requires combustion of a certain amountof a fuel for generating the reforming heat; a further amount of fuel istypically used to produce steam and to power steam turbines which drivemachines such as pumps or compressors, including the air compressors andthe gas compressors which raise the pressure of the generated make-upgas to the pressure of the ammonia synthesis.

For this purpose, the common approach of natural gas-based plants is touse a portion of the natural gas feedstock as fuel. Around 30-40% of thetotal consumption of natural gas is typically used as fuel, inparticular for firing of the primary reformer. The plant may alsoinclude natural gas-fired auxiliary boilers to generate the additionalsteam required by the process.

EP 2 662 327 describes a process for the production of ammonia wherenatural gas fed to the plant is split substantially into two portions: afirst portion (named process fraction) is used as reactant for thereforming process and a second portion (named fuel fraction) is used forthe operation of the plant.

However, in recent times limitations of quantity of natural gas and anincreasing instability of international hydrocarbon markets haveemerged. As a consequence, synthesis gas plants in locations where thenatural gas is expensive and/or is available in a limited amount mustminimize the consumption of natural gas. Hence, the processes availabletoday may be too expensive to operate, especially where gas costs arehigh.

In order to tackle the problem of how to maintain or increase theproduction of synthesis gas facing a limited supply of natural gas, theinterest in alternative hydrocarbon sources technologies has beenincreasing. Among the available sources, coal is of great interestthanks to its wide availability and inexpensiveness, representing afeasible alternative feedstock for the production of hydrogen-containingsynthesis gas and ammonia synthesis gas.

Coal-to-ammonia plants have been proposed, wherein coal is gasified athigh pressures and high temperatures, in the presence of steam and alimited amount of oxygen, and provides a synthesis gas containing mainlycarbon monoxide and hydrogen. Said gas is then purified through a seriesof steps including for example removal of particulates, sour gas shiftwhere CO is converted to CO2, removal of CO2 and H2S in an acid gasremoval unit.

A disadvantage of coal to ammonia plants is that they are much moreexpensive than gas based plants. One of drawbacks of said technique isthat the gas originated by the gasification of coal also containssignificant amounts of sulphur components (mostly H2S and COS) and otherimpurities (including chlorides, HCN, ammonia, metals), which must becompletely removed in order to use said gas as process gas. However, thefront-end of an existing ammonia plant is generally not able to processsuch coal-based process gas without an extensive and expensiverevamping. Therefore, the prior art coal-to-ammonia technology requiresa high investment cost and is not attractive for revamping of existingplants.

SUMMARY OF THE INVENTION

The invention aims at overcoming the drawbacks of the prior art as abovediscussed. In particular, the invention proposes to reduce the totalnatural gas consumption and simultaneously to produce ahydrogen-containing synthesis gas pure enough so as to avoid extensivepurification treatments, which would imply the process equipment beingexposed to detrimental substances; to provide a more environmentallyfriendly use of coal and other carbonaceous feedstock as energy source;to save capital costs and reduce energy consumption.

These objects are achieved with a process for the production of ahydrogen-containing synthesis gas from a natural gas feedstock accordingto claim 1, and a plant and method of revamping according to theattached claims. Preferred embodiments are disclosed in the dependentclaims.

Said process comprises the conversion of natural gas into a product gaswhich is then suitably purified. At least part of a heat input to saidprocess is provided by combustion of a fuel and at least part of saidfuel is a gaseous fuel obtained by the conversion of a carbonaceousfeedstock.

Natural gas is still used as process feedstock for the production of thehydrogen-containing synthesis gas, while fuel is at least partiallysupplied by the conversion of the carbonaceous feedstock. Accordingly,one of the major advantages of the invention is that the natural gaspreviously used as a fuel can be redirected for use as processfeedstock.

Said heat input may include a process heat, e.g. of a primary reformer,and/or heat for production of steam to drive steam turbines forcompressors, pumps or the like.

Said conversion of the carbonaceous feedstock denotes partial oxidationreactions carried out in the presence of an oxygen-containing stream andusually of water or steam. Preferably the gaseous fuel from saidconversion contains carbon monoxide and hydrogen, being suitable forreplacing at least partially the natural gas-based fuel commonly used inthe prior art.

Said carbonaceous feedstock in some embodiments is in solid or liquidform. A solid or liquid feedstock preferably comprises at least oneamong coal, lignite, coal-derived coke, petroleum coke and a liquid suchas heavy fuel oil.

The above are available at moderate cost in many geographical areas andin such areas represent an economically viable alternative to naturalgas fuel. The conversion of a solid or liquid carbonaceous feedstockinto a gaseous fuel is also referred to as gasification.

According to other embodiments, said carbonaceous feedstock is gaseous.In such a case the conversion of said carbonaceous feedstock into asuitable fuel may be a partial oxidation, either catalytic or not.

Preferred applications of the invention include: a process for makingammonia starting from natural gas; a process for making ammonia and urea(ammonia/urea process); a process for making methanol; processes formaking other synthesis gas derived products, such as hydrogen or carbonmonoxide or Fischer-Tropsch products or oxo-alcohols or gasoline throughmethanol.

Conversion of natural gas (process gas) may include reforming or partialoxidation of said natural gas into a reformed gas or partially oxidizedgas, respectively.

Preferably said conversion of natural gas includes a reforming step in areforming section. Said reforming section may include a steam reformer.According to various embodiments, said reforming section may include atleast one of a primary steam reformer and a gas heated reformer (GHR),and optionally a secondary reformer, the latter being fed with air,oxygen or enriched air. In some embodiments, the reforming sectionincludes an auto-thermal reformer (ATR). A pre-reformer may also beincluded in any of the above embodiments.

In one of the embodiments of the invention, said reforming section onlycomprises a steam reformer i.e. a primary reformer without a subsequentsecondary reformer. Reforming performed solely in a primary steamreformer is also termed pure reforming. Preferably, pure reforming iscarried out at relatively low pressure (i.e. 10-30 bar at the reformercatalyst tubes outlet) and high temperature (i.e. more than 850° C. atthe reformer catalyst tubes outlet), in order to maximize the productionof hydrogen per unit of natural gas used for the process fraction.According to some embodiments, a step pre-reforming may be included inthe pure reforming.

Embodiments with pure reforming are advantageous in particular when thehydrogen-containing gas is used to make methanol or to make ammonia inan ammonia-urea plant. An ammonia-urea plant is where some or all of theammonia is further reacted with carbon dioxide to form urea.

The advantages of the pure reforming in combination with the inventionare discussed in the following paragraphs.

In the methanol production, it is desired that the make-up gas has amolar ratio between (H2-CO2) and (CO+CO2) equal to 2. Pure steamreforming, however, produces a gas having said ratio equal to 3, whichmeans that hydrogen is in excess and the syngas need to be balanced. Oneway of balancing the methanol make-up syngas is to combine the steamreforming with oxygen auto-thermal reforming, as known in the art. Theinvention provides a source of CO2 which can be used to this purpose.CO2 can be recovered from the effluent of the conversion process of thecarbonaceous feedstock and added to the make-up gas, to balance theexcess hydrogen. Hence, the invention allows producing a balancedmethanol make-up gas with a pure steam reforming, which is lessexpensive and consumes less natural gas (as process gas) than aconventional primary and secondary setup. In other words, the recoveredCO2 provides part of the carbon for the product methanol, thus reducingthe natural gas consumption as to the process fraction.

In the prior art, the excess hydrogen is recovered with the purge of thesynthesis loop and used as a fuel. By balancing the make-up gas, theinvention allows using the full amount of hydrogen as a process gas(i.e. to make methanol) instead of fuel, the necessary fuel beingfurnished by the conversion of a carbonaceous feedstock. It follows thatthe natural gas consumption of the whole plant (as amount of gas/tons ofmethanol produced) is reduced compared to a prior art with primary andsecondary reforming. For example the consumption may be 25% lower.

In the ammonia-urea production, the pure reforming would result in alack of CO2 for urea synthesis. The invention solves this drawbackthanks to the CO2 recoverable from the effluent of the conversionprocess of a carbonaceous feedstock, thus allowing the use of the puresteam reforming.

In other words, the conversion of a carbonaceous feedstock provides aconvenient route for additional CO2 production, hence a means of makingup said shortage of CO2 and permitting complete conversion of theammonia produced into urea.

Limitations of using pure reforming in an ammonia-urea plant, such aslack of nitrogen for the ammonia synthesis, lower overall energyefficiency and lower single train syngas capacity other than lack of CO2for urea synthesis, are fully compensated by the significant reductionin natural gas consumption as to process fraction.

Further ways to reduce the natural gas consumption, according to theinvention, include: increasing the steam-to-carbon ratio (e.g. to valueshigher than 3), reducing the purge gas leaving the synthesis loop of theammonia plant, installing an additional (e.g. third) water-gas shiftreactor or installing a hydrogen recovery unit (HRU). Preferably, butnot exclusively, the above features are combined with the performing ofa pure steam reforming

A further aspect of the invention relates to recovery of CO2. Removal ofcarbon dioxide from the fuel gas, for example by a washing process, canbe performed for purposes such as increasing the calorific value of saidfuel gas, or carbon dioxide sequestration.

Preferably, CO2 is directly recovered from a portion of the fuel gasgenerated in the conversion process of a carbonaceous feedstock.

According to preferred embodiments, said portion of the fuel gas issubjected to water-gas shift in order to maximise the recovery of CO2,by converting the carbon monoxide contained therein into carbon dioxide.Carbon dioxide is subsequently separated, e.g. in a washing unit.

Further advantages of operating water-gas shift are higher heating valueof the fuel gas after removal of the CO2 and greater safety owing to thereduced CO partial pressure in the fuel gas.

The above techniques for recovery of CO2 are applicable to allembodiments of the invention. When pure steam reforming is used, saidpure steam reforming is preferably followed by two steps of water-gasshift (high and low temperature shift), or by a near-isothermal mediumtemperature shift and optionally one step of methanation, in order tomaximize the recovery of carbon dioxide and the production of hydrogenper unit of natural gas used for the process fraction.

The gasification of a solid carbonaceous feedstock is advantageouslycarried out in a fluidized bed or in a transport reactor. A gasificationreactor where a solid or liquid feedstock is converted into a gaseousfuel is referred to as gasifier.

A further aspect of the invention is that a low temperature andlow-pressure gasifier can be successfully used. A lowtemperature/low-pressure (LT/LP) gasifier is understood as operating atno more than 1000° C. and no more than 20 bar.

In the prior art, said LT/LP gasifiers are known to suffer the drawbackof larger residual amount of unreacted methane (CH4) or other lighthydrocarbons (e.g. ethane, C2H6) in the effluent

Thanks to the invention, the overall efficiency is not affected by thisdrawback since the effluent of the gasifier is used as fuel (not asprocess gas) and said unreacted methane or other hydrocarbons increasesthe heating value. On the other hand, a LT/LP gasifier has a lower costand lower consumption of oxidant and coal for a given gas fuel output,compared to a gasifier of the same capacity but working at a highertemperature and/or pressure.

According to some embodiments of the invention, a methane-rich streamcan be separated from the gasifier effluent at appropriate points. Thismethane-rich stream can be recycled back as reforming feedstock (processgas) to further reduce the natural gas consumption, lowering theconsumption of gas as process fraction. According to other embodiments,said methane-rich stream is used to provide part of the fuel required todrive a gas turbine or gas engine. Gas turbines or gas engines operatingon a methane-rich gas are cheaper, more common and more efficient thanthose operating on syngas containing high percentages of H2, especiallyin the power range up to 50 MW which is typical of chemical productionfacilities.

The gasifier may be reduced in size by selecting a reforming sectionthat includes a gas heated reformer (GHR), such as a section including aprimary steam reformer, a GHR and a secondary reformer. By routing theeffluent of the primary or secondary reformer to the shell side of theGHR, heat is provided to the tubes of the GHR. Hence, less fuel isrequired for the primary steam reformer, which means lower amounts ofthe carbonaceous feedstock to be gasified, hence smaller dimensions ofthe gasifier. This is advantageous especially if the plant hascoal-fired auxiliary boilers.

According to further embodiments, a reduction in the fuel consumption isobtained by pre-heating the natural gas feed or the mixed feed (i.e.comprising steam and natural gas) of the primary reformer using heatavailable downstream the gasifier, either in a heat exchanger or in aGHR.

The gasifier can be operated with air or oxygen or a mixture thereof asthe oxidant. In case of oxygen or oxygen-enriched air, the heating valueof the produced fuel gas is higher than with air, and the flametemperature achievable by combustion of the fuel gas is higher.

Further aspects of the invention concern the treatment of the gaseousfuel before combustion, for example to remove impurities such as solidparticles, sulphur compounds, methane and carbon dioxide.

Solid residues from the gasification process, i.e. ash and unconvertedcarbon, are partly removed from the gasifier, while fine particulatematter still present in the gaseous fuel downstream the gasifier may beremoved in a cyclone or gas filter or by direct contact with water. Thegasifier may be advantageously of the ash-agglomerating type.

Sulphur may be contained in the carbonaceous feedstock (e.g. coal). Acertain amount of sulphur in the feedstock may cause the formation ofcompounds such as hydrogen sulphide (H.sub.2S) and COS during theconversion of feedstock into fuel. At least a partial removal of sulphurcompounds from the fuel gas is desirable for environmental reasons andto avoid sour condensation of the flue gas which attacks the exchangersurfaces in the reformer, fired heaters and boilers.

An advantage of the invention over the coal-to-chemicals process is thatthe tolerable amount of sulphur in the fuel gas is significantly greater(e.g. two orders of magnitude) than in synthesis gas (process gas),meaning that simplified sulphur removal techniques can be used. A simpleand cheap sulphur separation method such as in-situ desulphurization orwarm desulphurization process based on sulphur adsorption may be used,as better described below. Said processes would not be suitable for acoal-to-chemicals plant, because they would not meet the strict gaspurity requirements of the downstream sections.

In a particular embodiment of the invention, at least part of thesulphur contained in the carbonaceous feedstock is removed in situwithin the gasifier. The gasifier is additionally supplied with a streamof a suitable sorbent such as dolomite (MgCa(CO₃)₂) or limestone (CaCO₃)and an amount of sulphur present in the feedstock is absorbed by thesorbent typically in the form of calcium sulphide. The spent sorbent isdischarged from the gasifier for further treatment or disposal.

Besides the apparent simplification of the process, which does notrequire dedicated downstream treatments for sulphur removal, the in situdesulphurization also has the following advantages.

First, the gas is advantageously fired at a temperature above the dewpoint (e.g. at 300° C.) since the change in water vapor contained in theflue gas (derived from the residual water vapor in the fuel) is small.Cooling of the gas below the dew point (i.e. around 200° C.) wouldinstead require expensive equipment due to the formation of a sour andcorrosive process condensate containing sulphur compounds and NH4Cl. Inaddition, the condensate would contain higher hydrocarbons and alcohols,relatively difficult to eliminate from the water. According to saidembodiment, the above problems are avoided, which means higherreliability of the process and absence of significant adverse effects onthe downstream units.

Firing the gas from the gasifier at a temperature higher than the dewpoint is also more efficient due to the recovery of the gas fullenthalpy, on top of the combustion, Otherwise, at least part of thewater vapor latent heat would be lost.

According to another embodiment, sulphur compounds (mainly H2S and COS)in the gaseous fuel leaving the gasifier are adsorbed on a suitablesorbent (e.g. a metal oxide sorbent like zinc oxide). The sorbent ispreferably loaded and then regenerated in a circulating fluid bed. Saidprocess is preferably carried out at high temperatures, i.e. higher thanthe gas dew point, thereby obtaining the same advantages above. Thesulphur is separated for example as sulphur dioxide (SO2).

In a further embodiment of the invention, the gaseous fuel leaving thegasifier and containing sulphur compounds is contacted in a gas liquidabsorber, with a liquid able to separate the sulphur components from thegas, such as an amine solution. The rich liquid is discharged from theabsorber for external regeneration, with optional production ofsulphuric acid (e.g. by means of the Wet Sulphuric Acid process) or,more preferably, production of elemental sulphur by a catalytic sulphurrecovery process (i.e. Claus Process) or a suitable biological process.

In other embodiments, said gaseous fuel containing H2S is subjected to abiological process, which directly provides elemental sulphur.

The load of the above desulphurization processes may be reduced bycarrying out water-gas shift in sour conditions, which converts sulphurcompounds (e.g. COS) into hydrogen sulphide (H2S), or by firsthydrolysing

COS and then subjecting it to a “sweet” water-gas shift. Another relatedadvantage is that carbon dioxide is removed prior to combustion and CO2emissions into atmosphere are reduced.

As explained above, the gaseous fuel from the gasifier may also containa significant amount of unreacted methane (e.g. 5% molar or greater) andsmaller amounts of other hydrocarbons, such as e.g. ethane. Amethane-rich stream is advantageously separated from the fuel streamafter the desulphurization process by a suitable recovery process, suchas cryogenic separation or separation through membrane.

At least a portion of carbon dioxide may be removed from the gaseousfuel leaving the gasifier after the gasification step. CO2 recovery fromsaid gaseous fuel is easier than conventional CO2 recovery fromcombustion flue gas when some of the natural gas is used as fuel.

Said carbon dioxide can be addressed to specific uses. For example, asalready stated above, carbon dioxide can be used for the production ofurea in an ammonia-urea integrated plant.

The gaseous fuel, typically depleted in its sulphur content as describedabove, is fed preferably to one or more burners installed in one or moreof the following devices: a radiant section of a primary reformer; aconvective section of a primary reformer; a desulphurizer pre-heaterarranged to preheat said natural gas before desulphurization andsubsequent reforming; a process fired heater; an auxiliary steamgenerator; a steam superheater; an HRSG (Heat Recovery Steam Generator)cooling the exhaust of a gas turbine; a gas turbine (for powergeneration or for driving a machine such as a compressor).

In the embodiments featuring the removal of hydrogen sulphide within thegasifier, the partly desulphurized gas leaving the gasifier may be fedto one or more of the above mentioned users after a simplified treatmentincluding a cooling step and removal of entrained particles, withoutfurther treatment.

The invention also concerns a related plant and method of revampingaccording to the claims.

A method of revamping according to the invention provides that a plantfed with a natural gas feedstock and wherein said natural gas feedstockis split into a first fraction (process fraction) used as process gasand a second fraction (fuel fraction) used as fuel, is revamped byaddition of a section wherein a carbonaceous feedstock is converted(e.g. coal is gasified) to produce a fuel, and said fuel replaces atleast part of said second fraction of the natural gas. Preferably, saidat least part of said second fraction of natural gas, originally used asfuel, is redirected for use as process gas.

Some of the advantages of the invention have already been discussed. Amajor advantage is less requirement of natural gas compared to thecapacity in terms of the chemical product (e.g. ammonia or methanol).

One of the advantages of the invention is the use of a low-costhydrocarbon source, such as coal, for the provision of the fuel, whilethe more valuable natural gas is entirely directed to the process line.The investment cost for implementation of the coal gasification sectionis significantly lower than the investment cost for a coal-to-chemicalplant.

This advantage is important also for a revamping of an existing plant.For example, a coal-to-chemical approach would totally replace naturalgas with coal, leading however to a high investment cost for meeting thepurity requirements of process gas. The present invention provides thatcoal-derived fuel gas replaces the amount of natural gas originally usedas a fuel fraction (typically 30-40% of the input), which is of moreeconomic interest. Hence, the invention provides a rather inexpensiveway to drastically reduce the total natural gas consumption or increasethe capacity for a given (available) amount of natural gas.

Generation of coal-derived fuel via gasification is more advantageousthan simply providing a coal-fired boiler to replace the naturalgas-fired auxiliary boiler, for the following reasons:

-   -   more natural gas fuel can be replaced with coal-derived fuel        gas,    -   coal gasification can also be successfully applied to        stand-alone plants, where there is no, or minimal firing of the        auxiliary boiler,    -   when applied as a revamp, it does not require replacement of the        fired heaters or the auxiliary boiler.

A further advantage is the environmentally friendly use of acarbonaceous feedstock, such as coal. Impurities are removed in thegasification process or downstream thereof, and a relatively pure streamof fuel is provided. Said impurities mainly comprise sulphur which getsconverted to hydrogen sulphide and carbonyl sulphide, and othercompounds such as CO2, HCN, NH3 and metals. Removal of said impuritiesbefore combustion is advantageous being much easier and more practicalthan removal from flue gas of a coal boiler.

Another advantage of the present invention is that the gasifier can berun at relatively low pressure, since the typical fuel pressure requiredat the burners is 3 to 5 bar. This reduces the capital cost of thegasifier and downstream equipment, and the capital cost for compressionof the oxidant. Moreover, in oxygen-fired embodiments, oxygen with amoderate purity can be used thereby reducing the cost and consumptionfor air separation. The air separation unit may even be a PSA (pressureswing) or a VPSA (vacuum pressure swing) or membrane, and notnecessarily a cryogenic air separation unit (ASU). Moreover, the purityrequirements for the fuel are much less stringent than for syngas to beused in chemical synthesis.

The invention will be further elucidated by the following description ofpreferred embodiments thereof, given by way of non-limiting examples.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an illustrative scheme of the process for the production ofhydrogen-containing synthesis gas according to an embodiment of theinvention.

FIG. 2 is a scheme of the front-end section of an ammonia plantaccording to a first embodiment.

FIG. 3 is a scheme of the front-end section of an ammonia plantaccording to a second embodiment.

FIG. 4 is a scheme of an embodiment of the invention for ammonia-ureaprocess.

DETAILED DESCRIPTION

FIG. 1 illustrates a block scheme of a process for producing ahydrogen-containing synthesis gas according to an embodiment of theinvention.

Block 300 denotes a reforming section, preferably of an ammonia plant,where a natural gas feedstock 301 is converted into a gas mixture 302,which is purified in a purification section 500 to obtain a product gas303. The purification section 500 preferably comprises a shift section,a CO2 removal section (502, shown in FIG. 4) and optionally amethanation section.

Block 400 denotes a coal gasification section, where a coal feedstock401 is converted into a gaseous fuel 402 by a gasification process withan oxidant such as air or oxygen 403 and water or steam 404.

The gaseous fuel 402 provides at least part of the total amount of fueldirected to the reforming section 300. Accordingly, the total amount ofthe feedstock 301 required for a particular production rate of ammoniais reduced. Alternatively, a larger amount of the feedstock 301 isavailable for the process, namely for generation of the product gas 303,hence the production of ammonia may be increased. Optionally, a portionof said fuel may be still taken from the natural gas feed 301. Saidportion (also called fuel fraction) is represented with a dotted line304 in the figure.

FIG. 2 shows a subunit 100 of the front-end section of an ammonia plant.Said subunit 100 comprises two sections: a first section 101 for theproduction of a reformed gas 8 from a natural gas feedstock 1, and asecond section 102 for the gasification of a coal feedstock 21 and itsconversion into a gaseous fuel 35.

Said first portion 101 comprises a primary reformer 103, which is inturn divided into a radiant section 104 and a convective section 105; apre-heater 106 and a desulphurizer 107 which are positioned upstreamsaid primary reformer 103.

The natural gas 1 enters said pre-heater 106, where it is heated to afirst temperature, e.g. around 350° C., and subsequently is directed tosaid desulphurizer 107, resulting in a stream 4 of desuplhurized naturalgas. Said outlet stream 4 is mixed with superheated steam 5 generating astream 6 of process gas.

Said stream 6 is fed to the convective section 105 of the primaryreformer 103 and it is further heated to a higher temperature, e.g.around 500° C., in a heat exchange coil 108.

The heated stream 7 is subsequently fed to the radiant section 104 ofthe primary reformer 103, containing an array of tubes filled withcatalyst where the conversion into a hydrogen-containing synthesis gasis carried out. The radiant section 104 is provided with a series ofburners 201 generating the reforming heat for the aforementionedconversion.

The convective section 105 of the primary reformer 103 substantiallyrecovers heat from the flue gas generated by said burners, which leavesthe reformer 103 at line 210. In particular, due to the hightemperatures of said flue gas, the convective section 105 is mainly usedto superheat the steam and to heat the process air feed to the secondaryreformer (not shown in the figure). For these reasons, the convectivesection 105 is typically provided, besides the aforementioned heatexchange coil 108 for the feeding stream 7, with at least one steamsuperheater coil 109 and a heat exchange coil 110 for the process air.

FIG. 2 also shows an auxiliary boiler 111 separated from the reformingsection 103 and producing additional steam. It should be noted that thissetup is purely illustrative and several variants are possible.

As already said above, stream 35 of gaseous fuel is generated in asecond section 102 where the gasification of a coal feedstock 21 takesplace.

Said second section 102 comprises a gasifier 112 and a series ofpurification equipment for removing undesirable impurities, e.g. cycloneor gas filter 114 and hydrogen sulphide adsorber 117.

Said coal feedstock 21, an oxidant stream 22 and steam or water 23 arefed to said gasifier 112, where they react at a high temperature(typically around 1000° C. or higher) to produce a gaseous fuel 25containing, besides H₂ and carbon monoxide, impurities like sulphur,nitrogen and mineral matter.

A continuous stream 24 of ash and unconverted carbon is provided fromthe bottom of said gasifier 112 to prevent the accumulation of solids inthe gasifier 112 itself.

Said gaseous fuel 25 free of most solid particles leaves the gasifier112 from the top and is passed through a heat recovery unit 113. Saidheat recovery unit 113 typically comprises a high pressure steam wasteboiler and/or a high pressure steam superheater. In some lower costembodiments, the gasifier effluent can be cooled by water quench.

After waste heat recovery, the resulting cooled synthesis gas 26 flowsthrough said cyclone or gas filter 114, which removes fine particulatematter 27 still present in the synthesis gas 26. Removing fine entrainedsolids 27 is an important step as fine particles in the synthesis gasmay foul or corrode downstream equipment, reducing performance.

The resulting clean synthesis gas 28 leaves the cyclone 114 and flows toan arrangement of heat exchangers 115, where it is cooled with anoptional heat recovery to near ambient temperature and condensedunreacted steam 30 is removed in a separator 116.

Subsequently, the cooled gas 31 leaving the separator 116 enters saidabsorber 117, in which it is scrubbed with a solvent 32 in order toremove hydrogen sulphide. The lean solvent 32 is typically an aminesolution. Elemental sulphur may be recovered from this hydrogen sulphideby a suitable catalytic sulphur removal process (not shown in thefigure). The loaded solvent is removed as stream 33 for externalregeneration.

Said removal of hydrogen sulphide in the absorber 117 may optionally becarried out by means of a biological process.

The scrubbed gas 34 mainly containing CO and H2, leaving the top of theabsorber 117, is optionally reheated in a heat exchanger 118 resultingin a heated stream 35.

Said stream 35 represents the fuel gas which provides the fired heatingfor the operation of the plant.

More in detail, referring to FIG. 2, said stream 35 fuels the burners201 of the radiant section 104 and, if present, the burners 200 of thedesulphurizer preheater 106, the burners 202, 203 of the convectivesection 105 and the burners 204 of the auxiliary steam generator 111.

According to FIG. 2, the fuel 35 is split into portions from 10 to 14,each supplying one of the aforementioned burners. In particular:

-   -   portion 10 fuels the burner 200 of the desulphurizer preheater        106;    -   portion 11 fuels the burner 201 of the radiant section 104;    -   portion 12 fuels the burner 202, provided to control the        temperature of the stream 6 fed to the convective section 105;    -   portion 13 fuels the burner 203, provided to control the        temperature of the superheated steam generated in the coil 110        of the convective section 105;    -   portion 14 fuels the burner 204 of the auxiliary steam generator        111.

FIG. 3 shows another embodiment of the present invention, and thecomponents are indicated by the same reference numbers.

The gasifier 112 is additionally supplied with a stream 36 of sulphursorbent, typically limestone, in order to remove most of the sulphurpresent in the coal feedstock 21.

The spent sorbent is discharged from the bottom of the gasifier 112together with ash and unconverted carbon in stream 24.

After passing through a heat recovery unit 113, a cyclone 114, thesynthesis gas stream 28 substantially free of sulphur and solidparticles is used as fuel and supplied to the burners.

FIG. 4 discloses another embodiment of the invention for implementationin an ammonia-urea plant. The syngas 303 is a make-up gas for synthesisof ammonia which is converted into ammonia 601 in an ammonia synthesissection 600. At least some or all of the ammonia 601 is used in a ureasection 602 for the synthesis of urea 603 with a carbon dioxide feed604.

A first portion 605 of the total CO2 requirement 604 for conversion ofthe ammonia into urea comes from the CO2 removal unit 502, typicallycomprising an MDEA or potassium carbonate washing unit, forming part ofthe purification section 500 of the reformed gas 302.

A second portion 606 of carbon dioxide is obtained from a portion of thefuel 402, i.e. from the gasification of coal. Said second portion 606 isa more substantial part of the total CO2 requirement 604 when thereforming section 300 only comprises a primary steam reformer and mostor all the ammonia is converted to urea.

More in detail, said portion 402 is directed to a shift reactor 608 toconvert the carbon monoxide contained therein into carbon dioxide. Theso recovered carbon dioxide is separated, for example in a washing unit609, and mixed with said first portion 605 to form the above mentionedfeed 604. Desulphurization of 402 is not shown.

The remaining portion 411 of the fuel 402 is sent to the reformingsection 300.

Example 1

An integrated ammonia/urea plant based entirely on natural gas as feedand fuel produces 2200 tonnes/day of ammonia of which approximately 85%is converted into urea, of which the production is accordingly 3300tonnes/day. Total energy requirement for the integrated plant, which iscompletely supplied in the form of natural gas, amounts to 5.2 Gcal LHVbasis per tonne of urea product, amounting in total to 715 Gcal/h. Ofthis total natural gas import, 3.1 Gcal/tonne (426 Gcal/h) is requiredas process feed for the steam reforming process, with the balance of 2.1Gcal/t (289 Gcal/h) used as fuel in the steam reformer and for thegeneration and superheating of high pressure steam.

The whole of this natural gas consumption as fuel can in principle bereplaced with a fuel gas generated from coal in a gasification facilityas described herein. However it is assumed that due to miscellaneouslosses the total LHV heating value required would be 10% higher (318Gcal/h) after conversion from all natural gas firing to all coal-derivedfuel gas. A fuel gas stream having a total LHV heating value of 318Gcal/h can typically be produced by gasification of approximated 75tonnes/h of bituminous coal (dry ash-free basis) at approximately 10bar/1000° C. in a fluidized bed gasifier requiring around 45 tonnes/h of95% purity oxygen.

By contrast a revamp of the 2200 tonnes/day ammonia plant forming partof an integrated ammonia/urea plant so as to use coal as processfeedstock would require gasification of approximately 110 tonnes/h ofbituminous coal (dry ash-free basis), typically at 50 bar with around 95tonnes/h of oxygen at around 60 bar—requiring a much larger capitalinvestment than the coal gasification scheme above. Moreover import of amaterial amount of high pressure steam from an external boiler plant(assumed to be coal fired) would be necessary to ensure sufficient steamand mechanical power for the ammonia plant and the downstream ureaplant.

Example 2

In a plant for the methanol synthesis, whereby the gas productionprocess is based on a pure steam reformer, 93.2% of the natural gas feedis required as process feed, with the balance of 6.8% used as fuel. Thetotal gas consumption for methanol production according to this processroute is around 7.4 Gcal/MT, based on the natural gas LHV.

Application of a first embodiment of the invention allows replacing thefuel fraction, which is 6.8%. Hence, it allows reducing the natural gasconsumption to 93.2% of the original value, i.e. 6.9 Gcal/MT based onthe gas LHV.

The amount of natural gas used as process feed can be drasticallyreduced by application of another embodiment of the invention, i.e.adding CO2 recovered from the gasifier effluent to the primary steamreforming. Accordingly, only 74.3% of the total original amount ofnatural gas is needed as process feed, or 5.5 Gcal/MT. The fuel fractionis produced by the gasifier. Hence, the gas consumption is reduced bymore than 25%, compared to the original value of 7.4 Gcal/MT ofmethanol.

It is worth considering that in a methanol synthesis plant according tothe art, whereby the syngas generation is based on a primary steamreformer followed by an oxygen auto-thermal reformer (i.e. based oncombined reforming), the total natural gas consumption is 7.0 Gcal/MT.This value is still 20% higher than the consumption value achieved bythe embodiment described above.

The invention can be applied also to a methanol plant based on combinedreforming.

What is claimed is:
 1. A method for revamping a plant for the productionof a hydrogen-containing synthesis gas, said plant comprising aconversion section, at least one fired device producing heat for saidconversion section and a fuel line being directed to said fired device;said plant being fed with a natural gas feedstock and said natural gasfeedstock being split into a first fraction used as process gas in theconversion section and a second fraction used as fuel and directed tosaid fired-device; wherein: a gasification section including a gasifierfed with a carbonaceous feedstock is added to said plant, wherein saidcarbonaceous feedstock is solid or liquid; said gasification sectionbeing arranged to produce at least part of said fuel directed to saidfired device, replacing a corresponding part of said second fraction ofnatural gas, wherein the conversion section includes a reformingsection, and the effluent of said gasifier is used as fuel, not as aprocess gas, and provides at least part of the total amount of fueldirected to said reforming section.